1. Field of Invention
The present invention relates to a soft nitrided gear and a method for fabricating the same, or more particularly to a nitrided gear having a comparatively thin compound layer and a comparatively thick diffusion layer and a method of fabricating the same.
2. Description of Related Art
Generally, high-strength gears, especially gears used for an automatic transmission, require a high abrasion resistance and a high strength. Of all such gears, a ring gear coupled to an input shaft or an output shaft of a planetary gear has such a length of action (the number of teeth in mesh per output rotation) that it has a considerable effect on the gear noises.
Conventional gears are known that are intended for a higher surface abrasion resistance, a higher breakage resistance and a higher fatigue resistance to further improve the dimensional accuracy by reducing thermal strain. An example is disclosed in JP-A-8-165556, in which a gear made of steel containing C, Si, Mn, Cr, Mo, Al, N and V is soft nitrided with a gas having a gas volume composition ratio of RX/NH.sub.3 ranging from 0.5 to 1.5 at a processing temperature ranging from 550 to 650 .degree. C., followed by forming a porous layer having a thickness of 10 .mu.m or less. The RX gas has the composition 23 vol. % CO, 30 vol. % H.sub.2 and 47 vol. % N.sub.2.
As a result, a sufficient surface hardness and a sufficient hardened depth are obtained, and at the same time the thickness of the porous layer is limited such that the problem of separation of the outermost compound layer (reduction in pitching resistance) can be solved.
The porous layer of the above-mentioned conventional gear, however, is as thick as 10 .mu.m. Accordingly once the porous layer is separated, the surface of the gear (meshed surface) tends to become rough to a comparatively high degree, which may deteriorate (increase) gear noises.
In view of this, the object of the present invention is to provide a gear and a method of fabricating the gear in which the porous layer is suppressed as far as possible while maintaining a diffused layer by selecting specific soft nitriding conditions, and a sufficient pitching resistance, a sufficient surface hardness and a sufficient hardened depth are obtained to secure an abrasion resistance, a breakage resistance and a fatigue resistance, while at the same time suppressing further deterioration of gear noises.